A common environmental measurement measures the concentration of a hydrocarbon contaminant, such as oil, in a sample, such as a sample of water, or a sample of a particulate matrix, such as soil. Several ASTM methods have been released for measuring the concentration of oil in water, including D7678-11, D3921, and D7066-04. These methods involve adding a defined quantity of a solvent to a defined quantity of a sample of water and agitating the resulting mixture. The solvent has low solubility in water and is a good solubilizer of the contaminant. The mixture is left to separate, and a defined quantity of a solution composed of the contaminant extracted from the water dissolved in the solvent is drawn off and put in the measurement cell of an infrared spectrometer. The spectrometer measures the absorption of infrared light by the solution at a defined infrared wavelength or in a defined infrared wavelength range. Software associated with the spectrometer converts the absorption measurement to a measurement of the concentration of the hydrocarbon contaminant in the sample. Alternatively, a method exists for liquid-liquid extraction followed by gravimetric determination, EPA 1664. Each of these methods can be used to measure of total oil and grease (TOG) and, after filtration, total petroleum hydrocarbons (TPH) and total grease (TG). TPH is highly regulated with respect to water discharge.
Solvent extraction methods originally used such solvents as carbon tetrachloride and carbon disulfide as the solvent. However, these solvents are harmful to the central nervous system and are harmful in other ways, so that their use was discontinued. Present-day solvent extraction methods use solvents that are either halogenated or contain volatile organic hydrocarbons (VOC). Halogenated solvents are expensive to purchase and to dispose of. Moreover, halogenated compounds are typically ozone depleting and are often toxic to marine life if they are released in waste water. Petroleum-based solvents are considered by many governments to contribute to global warming.
The newest technique specified in ASTM D7678-11 uses cyclohexane as the solvent. Cyclohexane has the disadvantage that it is also a hydrocarbon. ASTM D7678-11 measures the infrared absorbance of methyl groups (CH3) in hydrocarbons, since cyclohexane has no methyl group absorbance. The problem is that infrared absorption by the methyl group used in the cyclohexane methods is typically four times weaker than infrared absorption by the methylene groups (CH2) that can be used with other solvents. The weaker absorption results in more variability in the results at low contaminant levels. Furthermore, the solvents used in ASTM D7678-11 (cyclohexane) and EPA 1664 (hexane) are both highly flammable, unhealthy to breathe (personnel exposure limits must be complied with), their vapors may have greenhouse gas contributions, and are subject to U.S. Federal VOC regulations.
Accordingly, what is needed for use in solvent extraction methods is a solvent that is non-toxic, has low to moderate flammability, is inexpensive to purchase and to dispose of, and that does not harm the environment.